1. Field of the Invention
The present invention relates to a cross flow microfilter.
2. Description of the Background
Cross flow microfilters serve to separate or enrich suspended or emulsified particles from or in a solution such as finely dispersed precipitation products in chemical processes, microorganisms or cellular components. Either the filtrate or the solution enriched with suspended or emulsified particles may be the desired product in different applications.
The filters utilize porous membranes which are primarily made of plastic, sintered glass or sintered metal. These membranes are used as the filtering elements.
The membranes are made of various materials having their own characteristic pore size distributions. Typical membrane pore size distributions for, e.g., polypropylene materials lie between about 0.5 .mu.m and 1 .mu.m and for sintered glass between 3 .mu.m and 10 .mu.m.
The total porosity is also material specific and lies typically between about 20% and 50% for sintered metal, at about 60% for sintered glass and between about 60% and 85% for plastics such as low pressure polyethylene, polypropylene, polytetrafluoroethylene (see, e.g., A.E. Ostermann, Cross Flow Microfiltration In Industrial Use--Overview Of Present Materials And Examples For Use In Connection With Waste Water Recycling And Chemical Process Technology, lecture materials for Seminar No. 116125, Technical Academy, Wuppertal, May 1985).
Such membranes have relatively high flow resistance in spite of their high total porosity. The reduction of the flow resistance by reducing the thickness of the membranes is possible only within certain limits for various reasons. A minimum thickness must be provided, particularly in order to assure with sufficient certainty that the membrane will not be transversed over its entire thickness by individual channels whose openings are too large. Another reason for this is that large opening channels inevitably occur locally in membranes on the basis of statistical pore size distributions specific to the manufacturing process.
The membrane must also have sufficient thickness in order to attain a minimum strength. So-called nuclear track membranes such as the ones being manufactured by the firm Nuclepore have uniformly sized pore openings. These membranes also have relatively high flow resistance due to their poor transparency or ratio of the sum of the pore cross-sections to the total surface of the membrane.